Beating Between the Quasi-periodic Oscillations and the X-ray Pulsations in 4U 1626-67

Details Beating Between the Quasi-periodic Oscillations and the X-ray Pulsations in 4U 1626-67 Beating Between the Quasi-periodic Oscillations and the X-ray Pulsations in 4U 1626-67

Jan 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998baas...30..761k&link_type=abstract

American Astronomical Society Meeting, 191, #126.02

Astronomy and Astrophysics

Astronomy

Scientific paper

The low-mass X-ray binary pulsar 4U 1626-67 shows 0.048 Hz quasi-periodic oscillations (QPOs) and red noise variability as well as coherent pulsations at the 0.130 Hz neutron star spin frequency. Power density spectra of observations made with the Rossi X-ray Timing Explorer show significant side bands separated from the pulsar spin frequency (and its harmonics) by the QPO frequency. These show that the instantaneous amplitude of the coherent pulsations is modulated by the amplitude of the QPOs. This phenomenon is expected in models such as the magnetospheric beat frequency model where some of the QPOs originate near the polar caps of the neutron star. In the 4--8 keV energy range, however, the lower-frequency side bands are significantly stronger than their higher-frequency complements. The presence of enhanced lower-frequency side bands is not predicted by the magnetospheric beat frequency model of QPOs. We suggest that the QPOs are instead produced by a structure orbiting in the accretion disk at the QPO frequency. This structure re-processes the pulsar beam and produces the excess variability observed in the lower-frequency side lobes. We find no evidence that the red noise variability modulates the amplitude of the coherent pulsations; this is in contrast to the situation in some high-mass X-ray binary pulsars.

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